Two Key Mechanisms of Large-Scale Cross-Shelf Penetrating Fronts in the East China Sea: Flow Convergence and Thermocline Undulation

Cross-shelf penetrating fronts (CPFs) induce significant cross-shelf exchange of water properties and nutrients, and thus are important to coastal environments. In this study, the characteristics and mechanisms of realistic large-scale CPFs in the East China Sea in summer were investigated based on a data assimilative model. The model reproduced CPFs matched well with satellite observations. Although the cross-shelf currents were predominantly offshore off the Zhe-Min Coast, only three strong large-scale CPFs occurred in the summer of 2014. The three-dimensional structure of CPF in the model was similar with that observed in previous research. Two different mechanisms were responsible for the formation of observed CPFs. Two CPFs formed as a result of the convergence of the Taiwan Warm Current (TWC) and the Zhe-Min Coastal Current (ZMCC), while the other one was caused by the undulation of thermocline. Heat budget analysis suggests that the undulation of thermocline was caused by horizontal and vertical advection. Sensitivity experiments suggest that southerly wind relaxation and tidal forcing are indispensable conditions for CPF formation. Tidal forcing makes the axis of the ZMCC shift offshore by similar to 50 km, so that the ZMCC could impinge right against the axis of the TWC. The relaxation of the southerly winds allows the ZMCC to extend southward. Southerly wind relaxation in summer is mostly associated with tropical cyclones. Without winds and synoptic variation of the TWC, CPFs form periodically due to the strengthening of the ZMCC during neap tide period. Coastal ocean currents generally follow isobaths on timescale longer than one day. Cross-shelf currents induce significant exchange of nutrients and dissolved materials and thus are important to coastal environments. Cross-shelf penetrating fronts are observed as large-distance cross-shelf frontal intrusion from satellite ocean-color observations. They frequently occurred in the East China Sea, but their key mechanisms are not very clear. Based on results of a numerical ocean model, we investigated several realistic cases of penetrating fronts in the summer of 2014 and revealed two major mechanisms. One mechanism is the convergence of two ocean currents that flow in the opposite direction. The other mechanism is associated with the thermocline undulation, which will induce cross-shelf currents through dynamic adjustment. The impact of several important factors which include tide forcing, winds, synoptic variation of shelf currents and river discharge were also evaluated with numerical experiments. The results show that tidal forcing and the relaxation of southerly winds are indispensable condition for penetrating fronts. The relaxation of southerly winds in summer is usually associated with the influence of typhoons. The revealed mechanisms advance our understanding of cross-shelf currents in coastal seas. Flow convergence and thermocline undulation are two major mechanisms of the penetrating fronts Tidal forcing and relaxation of southerly wind are indispensable conditions for penetrating front formation The relaxation of southerly winds in summer is mostly associated with tropical cyclones